Gulf of Maine Seasonal Sea Surface Temperature Update

Seasonal Patterns in Sea Surface Temperatures

Author

Adam Kemberling

Published

August 24, 2023

About the Updates:

Over the past decade, scientists at the Gulf of Maine Research Institute have led a body of research that highlights the rapid pace of warming in the Gulf of Maine. To keep you informed, we share seasonal updates about conditions in the Gulf of Maine.

Note About the Data: The figures in this report are created using remotely-sensed satellite data as part of publicly funded research efforts. Satellite SST data was obtained from NOAA’s National Center for Environmental Information (NCEI), with all maps and figures displaying NOAA’s Optimum Interpolation Sea Surface Temperature Data.

The Gulf of Maine Region

For analyses like these, it is important to be clear about the spatial extent that “defines” the Gulf of Maine (Figure 1), as different borders could produce different results. The spatial domain we use as the “Gulf of Maine” is displayed below. This area is consistent with previous seasonal and annual Gulf of Mainereports GMRI has produced.

Season Highlights

For this seasonal report we present an analysis of SST for summer (June 1st, 2023 - August 31st, 2023). During this season, the average SST for the Gulf of Maine was 60.66°F, making it the 8th hottest summer on record for the period of 1982-2023 — the period over which the satellite data used are available. This seasonal average temperature is 1.93°F above the 1991–2020 CRP spring average of 58.73°F.

Weekly Temperatures

In (Table 1), we highlight how the SST for each week this summer compares to the 30-year CRP period (i.e., climatological averages from 1991 through 2020) for the area shown in Figure 1.

The observed SST, long-term average SST trends, and SST anomalies (i.e., departures from the long-term average SST) are shown. Departures from the long-term average were 1°F or more above normal during March & April, with temperatures in May falling closest to the long-term average.

Table 1:
Observed, climatological average, and deviation from the climatological average (i.e., temperature anomaly) for SST at a weekly resolution in the Gulf of Maine during spring 2023.
One-Week Period	Observed Temperature		Climatological Average		Temperature Anomaly
Weekly Averaged Sea Surface Temperatures - Summer
(June 1st, 2023 - August 31st, 2023)
One-Week Period	°F	°C	°F	°C	°F	°C
Jun 01 - Jun 03	53.80	12.10	50.00	10.00	3.78	2.10
Jun 04 - Jun 10	52.10	11.20	51.30	10.70	0.81	0.45
Jun 11 - Jun 17	54.00	12.20	53.10	11.70	0.98	0.55
Jun 18 - Jun 24	55.50	13.10	54.90	12.70	0.63	0.35
Jun 25 - Jul 01	58.90	14.90	56.80	13.80	2.11	1.17
Jul 02 - Jul 08	60.40	15.80	58.50	14.70	1.90	1.06
Jul 09 - Jul 15	62.50	17.00	59.90	15.50	2.59	1.44
Jul 16 - Jul 22	65.10	18.40	61.10	16.20	3.91	2.17
Jul 23 - Jul 29	66.50	19.20	62.10	16.70	4.43	2.46
Jul 30 - Aug 05	64.70	18.20	62.80	17.10	1.93	1.07
Aug 06 - Aug 12	63.90	17.70	63.30	17.40	0.62	0.34
Aug 13 - Aug 19	64.70	18.20	63.40	17.40	1.35	0.75
Aug 20 - Aug 23	63.80	17.70	63.30	17.40	0.52	0.29
Data Source: NOAA OISSTv2 Daily Sea Surface Temperature Data.
Climatological Reference Period: 1991-2020.

Monthly Statistics

Table 2 shows monthly average SST for 2023, where we see each month was nearly 2°F or more above the 1991-2020 CRP. July showed the largest deviation from the long-term climatological average, with an average SST anomaly of 3.12°F.

Table 2:
Observed, climatological average, and deviation from the climatological average (i.e., temperature anomaly) for SST at a monthly resolution in the Gulf of Maine during spring 2022 (defined as March 1 through May 31).
	All-Year Rank	Observed Temperature		Climatological Average		Temperature Anomaly
Monthly-Averaged Sea Surface Temperatures - Summer
(June 1st, 2023 - August 31st, 2023)
	(1982-2023)	°F	°C	°F	°C	°F	°C
Jun	9	54.90	12.70	53.50	11.90	1.38	0.77
Jul	4	63.60	17.50	60.50	15.80	3.12	1.73
Aug	12	64.30	17.90	63.20	17.40	1.04	0.58
Data Source: NOAA OISSTv2 Daily Sea Surface Temperature Data.
Climatological Reference Period: 1991-2020.

Seasonal Trends and Anomalies in Context

The Gulf of Maine is an area of particular interest to the scientific community because of the remarkable rate of warming it has experienced in recent years coupled with its importance as a major driver for the regional economy.

When looking at average summer temperatures from the beginning of the satellite record in 1982 Figure 2, there is a clear long-term increase, with SST’s warming at a rate of 1.04°F per decade. This rate of seasonal spring warming is ~4x the rate that global ocean temperatures are warming (0.27°F per decade).

Figure 2: Average annual spring SSTs in the Gulf of Maine from 1982 through 2022 (black dots). The orange line indicates the trend for the full time series for the Gulf of Maine. The blue line indicates the trend for the full time series for the global oceans.

The unprecedented levels of warmth over the past decade or so are consistent with what researchers believe has been a distinct regime shift in terms of the major influences on SSTs in the Gulf of Maine. The drivers of this (e.g., a potential weakening of the Atlantic Meridional Overturning Circulation, widening of the Gulf Stream, changes in the characteristics of the Labrador Current) have been well-documented in the peer-reviewed literature, including through research by GMRI scientists.

How Does this Summer Compare?

2023 is the 8th warmest summer season observed in the Gulf of Maine during the 41 years we have satellite data to analyze. The top 5 warmest summer seasons have all occurred in the last decade.

Figure 3: A ranking of the 5 warmest spring seasons for the Gulf of Maine in the satellite record (1982-2023). 2023 was the second warmest spring on record; the past three springs were each the warmest until that point; and the five warmest springs have all occurred since 2015.

Marine Heatwave Conditions

The most commonly used definition of a “marine heatwave” (MHW) is when daily average SSTs exceeded the 90th percentile of a climatological (i.e., 30-year) average for at least 5 consecutive days. Gaps of 2 days or less in this threshold do not constitute a break in the MHW event.

Using this broadly accepted definition, the Gulf of Maine has experienced MHW conditions for 19% of this summer. May was the only month this Spring where where SST was below the threshold of a MHW (90th Percentile).

Figure 4: A timeseries of marine heatwave (MHW) conditions in the Gulf of Maine extending from January 1, 2023 through May 31, 2023. Black lines represent the long-term (i.e., 1991 – 2020) average SST, the 10th percentile, and 90th percentile for a given day in the Gulf of Maine are labelled to indicate climatological reference points; a solid line (red for marine heatwave or blue for a non-event) indicate the observed SST this year; red and blue shading illustrates how far the observed SST falls from the climatological mean.

Presenting SST conditions in terms of anomalies (Figure 5) as opposed to absolute values (Figure 4) illustrates in greater detail the magnitude of MHW conditions throughout the summer. The most extreme daily SST anomalies occurred in March, with temperatures exceeding 5°F above the climatological average.

Figure 5: A timeseries of daily average SST anomalies in the Gulf of Maine (solid red line) compared to marine heatwave (MHW) conditions (dashed black line) in the Gulf of Maine for the period January 1, 2023 through May 31, 2023.

Heatmap of Temperature Anomalies and Heatwave Events

Looking at the full record of daily SST anomalies in the Gulf of Maine (Figure 6), the distinct thermal regime shift beginning around 2010 is evident. Indeed, since 2012, the Gulf of Maine has experienced far more persistent MHW conditions (indicated by solid black lines) than at any other point in the satellite record.

Figure 6: Heat map of daily SST anomalies from the beginning of 1982 through spring 2023. Not only do more large warm anomalies (darker reds) appear more frequently in recent years, but the frequency, duration, and intensity of marine heatwave events (black lines) in the Gulf of Maine has become more pronounced in the past decade.

The Visual Impact of a New Climate Reference Period

The identification and communication surrounding marine heatwaves is an area of active scientific discussion. Using our heatmap figure (Figure 6) as a visual guide, we can compare what the competing MHW methodologies say is the MHW record for the region. The transition to a full climatological reference period that covers all years (1982-2022) and the removal of the long-term trend brings a focus onto only the most “extreme” events. Using the slider below, we can showcase what a transition from the fixed-baseline methodology used in this report (and previous reporting), to an approach that removes long-term trends.

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From this view we can reveal that the transition has removed the majority of marine heatwave events, solving a communication challenge of the “year long heatwave”. By adopting this new method scientists have succeeded in identifying the extreme temperature swings, but there is ongoing debate on when to apply this method, and on some of the unintended consequences of doing so. Among the concerns that scientists have voiced is that this new method retroactively changes the history of past events by accommodating all the more recent (often more extreme) conditions. This makes it difficult to preserve the significance that these events had during their own times.

A second major concern surrounds removing the “trend”, and methodological choices scientists must make to achieve this. This new MHW methodology portrays this step in its simplest form, a linear trend. This structure may not be appropriate across all areas and at varying lengths of temporal coverage. The selection choices of the years to include or exclude and how best to model the observed trends create more “knobs” for practitioners to tune and with them more challenges for clarity of communication. To learn more about these trade-offs and the discussion around Marine Heatwaves, please check out our technical report on the subject.

Spatial Distribution of Seasonal Anomalies

From a spatial perspective, the Gulf of Maine and (most) surrounding areas experienced above average SSTs during summer 2023, but the warmest patches were to the south and east of Georges Bank, mostly outside the domain analyzed in preceding sections. The highest seasonally averaged anomaly of any location above was 5.68°F - along the southern edge of the domain analyzed.

Figure 9: Map of average SST anomalies for each grid cell in the satellite record for spring 2023. The box outlined by the black dashed line denotes the region of study for the analysis (see Figure 1). Darker red regions indicate warmer anomalies.

Monthly Temperature Anomalies

Average monthly SST anomalies are shown in Figure 10. The warmest anomalies were observed in March and April and were largely confined to areas just beyond the region of study (i.e, beyond the continental shelf where the Gulf Stream and Labrador Current have significantly more influence on oceanic conditions).

Figure 10: This series of maps shows the average monthly SST anomaly for March 2023, April 2023, and May 2023. The box outlined by the black dashed line denotes the region of study for the analysis. Darker red regions indicate warmer anomalies.

A Note on Data Sources:

NOAA_ERSST_V5 data provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their Web site at https://psl.noaa.gov/data/gridded/data.noaa.ersst.v5.html.

NOAA High Resolution SST data provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their Web site at https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.highres.html.

Citing This Work

If you would like to cite this report, please use:

Gulf of Maine Research Institute. 2022. Gulf of Maine Warming Update: Summer 2023

A work by Adam A. Kemberling

Akemberling@gmri.org